1. Field Of The Invention
The present invention relates to marginless multilayer electrical components, such as capacitors, and to methods for their manufacture.
2. The Related Art
There are numerous ways to construct electrical components such as ceramic capacitors. In a conventional multilayer capacitor-making process, green (unfired) ceramic sheets having thicknesses of several hundred micrometers or less are prepared by tape casting, for example. Noble metal electrodes are screen printed onto the green ceramic by means of a conductive ink. The metals used must have relatively high melting points and be nonreactive at elevated temperatures, to withstand the high temperatures needed to sinter the dielectric. As a result, expensive metals such as platinum or palladium are often used.
Thereafter sheets of green ceramic and electrodes are stacked on top of each other, with the electrodes staggered and partially overlapping each other, such that every other electrode extends to one side edge of the ceramic. Every electrode has one side edge which extends to one side edge of the ceramic, and an opposing side edge which remains within the ceramic. The region between the electrode side edge remaining within the ceramic and the side edge of the ceramic is known as an end margin.
In addition, the side margins are sometimes left between the front and back electrode edges and the front and back edges of the ceramic in order to protect the capacitor against undesired electrical shorting between electrode layers.
The stacks are then cut and fired at temperatures of up to 1350.degree. C. or higher, depending on the dielectric used, in order to properly sinter the ceramic dielectric. The ends of the device are then coated or terminated with a conductive metal or mixture, to connect the alternate electrodes.
One factor which determines the electrostatic capacity obtained from the capacitor is the area of overlapping between the electrode and the dielectric. The larger the area of overlap, the greater the capacitance. However, because of the presence of end margins and side margins, the total area of the ceramic layers cannot be made to contribute to the capacitance. In addition, the capacitive value of each chip is also dependent on the precision with which adjacent layers are oriented to one another.
The difficulties and costs inherent in the outlined manufacturing steps are multiplied when it is desired to produce capacitors or other devices to close tolerances.
For close tolerance operations, one method used is to manufacture capacitors with capacitance exceeding the desired value. After the capacitor is formed the overlap area of the electrodes is eroded or worn-away as by a sand blasting jet directed normal to the broad surface of the capacitor, so as to reduce capacitance to the desired value. These manufacturing steps will often significantly increase the cost of the capacitor.
U.S. Pat. No. 4,453,199 to Ritchie et al. describes a method of forming, by vapor deposition or the like, discrete electrode areas on an insulating substrate, and depositing a dielectric layer over the electrodes and the areas between the electrodes. A series of electrodes is formed over the dielectric in partial registry with the first set of electrodes, and the device is then diced to expose opposite edge portions of the resultant capacitors.
U.S. Pat. No. 4,771,520 to Tanaka discloses a method of producing laminated multilayer ceramic capacitors in which a stack of green sheets and electrodes is cut so as to expose the electrodes on the lateral surfaces, which are coated with a ceramic slurry. Then the resultant bodies are cut into chips and terminated on their opposing side surfaces.
U.S. Pat. Nos. 3,469,294 to Hayashi et al. and 3,457,614 to Tibol describe methods for making capacitors in which a metal electrode is anodized to form a dielectric layer, then another electrode layer is deposited on top of the dielectric layer.
It would be desirable to construct a marginless, close tolerance capacitor requiring fewer processing steps than the conventional methods outlined above. In addition, it would be desirable to replace precious metals used for electrodes with less expensive metals. It is to these types of objectives that the present invention is directed.